A Comparative Study of Phosphonate and Phosphorus-Free Antiscalant Efficiency by Static and Dynamic Methods. Do We Have Reliable Tools For An Adequate Reagent Selection?

Scale formation in the oil and gas industry, evaporation plants, reverse osmosis desalination processes, steam generators, boilers, cooling water towers and pipes is a serious problem, causing significant plugging of wells, pipe-lines, membranes, and increasing the production expenses [1,2]. A widely used technique for controlling scale deposition is an application of chemical inhibitors [1-4]. Commonly used commercial antiscalants are represented by organophosphonates and numerous modifications of polyacrylates (PA). Among these the organophosphonates are dominating recently at the World market [5]. At the same time phosphorus-based inhibitors are hardly biodegradable and persist for many years after their disposal, which leads to eutrofication problems. Phosphorus discharges are therefore regulated in many countries worldwide, and permissible limits are constantly decreasing.


Introduction
Scale formation in the oil and gas industry, evaporation plants, reverse osmosis desalination processes, steam generators, boilers, cooling water towers and pipes is a serious problem, causing significant plugging of wells, pipe-lines, membranes, and increasing the production expenses [1,2]. A widely used technique for controlling scale deposition is an application of chemical inhibitors [1][2][3][4]. Commonly used commercial antiscalants are represented by organophosphonates and numerous modifications of polyacrylates (PA). Among these the organophosphonates are dominating recently at the World market [5]. At the same time phosphorus-based inhibitors are hardly biodegradable and persist for many years after their disposal, which leads to eutrofication problems. Phosphorus discharges are therefore regulated in many countries worldwide, and permissible limits are constantly decreasing.
Increasing environmental concerns and discharge limitations have forced the scale-inhibitor chemistry to move toward "green antiscalants" that are readily biodegradable and have minimal environmental impact. Intensive efforts are applied recently to develop the "green" alternatives to organophosphonates and nonbiodegradable polyacrylates [1][2][3][4]. Among these novel inhibitors, such chemicals as polymaleates (MA), polyaspartates (PASP), polyepoxysuccinates (PESA), as well as their various derivatives, including co-polymers with PA are the most promising. It is important to note, that the new antiscalants should have acceptable levels of performance at a cost-effective dose rate. This requirement raises a problem of reliable tests, which permit a correct "old red" and "novel green" inhibitors efficiency comparison [6]. Indeed, most of the data published on CaCO 3 (CaSO 4 ) deposition are studied under hardly comparable conditions, e.g., different CaCO 3 supersaturationindex, brine composition, temperature, pH, measurement technique, etc. This leads to the quite opposite opinions on the relative Antiscalant's efficacy, reported by different research groups for one and the same set of reagents proposed for one and the same scale (see [1,7,8] and references there).  Table 1: impact of 10 mg•dm −3 inhibitor dosages on CaCO 3 scaling according to kinetic and static testament A full scale paper is in preparation paper is now underway: [8]. Specific Technology Group (STG) 31 on Oil and Gas Production-Corrosion and Scale Inhibition has elaborated a procedure of static laboratory antiscalants screening-NACE Standard TM0374-2007 [9]. These test methods are recommended only for ranking the performance of different scale inhibitors under laboratory conditions set by these methods. They are not intended to provide actual field treating rates. Surely, the scale inhibitor concentration required for a field application is likely to be different from that determined under these laboratory conditions. However, it is assumed, that for a particular set of reagents the ranking would be the same, and an inhibitor evaluation prior to final scale inhibitor selection is valid for the field use as well. Although particularly the NACE Standard is not very common, a lot of researchers use recently very similar approaches [1,7,8].

Regent
Present paper is therefore focused on the assessment of relative antiscalants efficacy against CaCO 3 scale formation predicted following NACE protocol, and their ranking found by kinetic experiments run under conditions close to those used at evaporation plants.
In any case PESA and MA-AA look more preferable than phosphonates for evaporation plants. Actually τ 1/2 seems to be a more adequate indicator than τ ind . The latter characterizes only the initial nucleation step, while the former is responsible for both: initial nucleation and further crystal growth kinetics. Anyhow the data given above demonstrates clearly that a lot of work is still needed to elaborate a system of laboratory tests for antiscalants in order to provide reliable assessment and selection on their way from laboratory to industry.

Conclusion
A comparison of static and dynamic laboratory testaments of scale inhibitors indicates for one and the same set of reagents rather conflicting results. Static test gives preference to the phosphonates ATMP and HEDP, while the dynamic oneto polymers PESA and MA-AA. Thus a lot of work is still needed to elaborate some conventional methods for the reliable, particular case-focused reagent efficiency prediction.